The communication system that is a successor to the Wideband Code Division Multiple Access (WCDMA) system, High-Speed Downlink Packet Access (HSDPA) system, High-Speed Uplink Packet Access (HSUPA) system and the like i.e. Long Term Evolution (LTE) has been established by 3GPP that is the standardization group of WCDMA (Release-8). As a radio access scheme in Release-8 LTE (hereinafter, referred to as REL8-LTE), the Orthogonal Frequency Division Multiplexing Access (OFDMA) system is specified in downlink, while the Single-Carrier Frequency Division Multiple Access (SC-FDMA)) system is specified in uplink.
The OFDMA system is a multicarrier transmission system for dividing a frequency band into a plurality of narrow frequency bands (subcarriers), and assigning data onto each subcarrier to perform transmission. The subcarriers are orthogonal to one another and arranged densely on the frequency axis, high-speed transmission is thereby achieved, and it is expected enhancing spectral efficiency.
The SC-FDMA system is a single-carrier transmission system for dividing the frequency band for each terminal, and performing transmission using frequency bands different from one another among a plurality of terminals. It is possible to reduce interference between terminals with ease and effectively, it is further possible to reduce variations in transmission power, and therefore, this system is preferable from the viewpoints of low power consumption in the terminal, wide coverage, etc.
Further, REL8-LTE specifies downlink reference signal structures. The downlink reference signals are used in 1) downlink CQI (Channel Quality Indicator) measurement for scheduling and adaptive control, 2) channel estimation for downlink coherent detection in user terminals (hereinafter, referred to as LTE terminals) supporting REL8-LTE, and 3) estimation of downlink propagation path state for cell search and handover. Defined as the downlink reference signals are a cell-specific reference signal, reference signal common in a plurality of cells, and specific reference signal for beam forming.
Furthermore, REL8-LTE specifies radio transmission methods (MIMO: Multiple-Input Multiple-Output) for improving the communication quality by providing each of a transmitter and receiver with a plurality of antennas (for example, Non-patent Document 1). The methods are divided into the case (single-user MIMO) where all concurrently transmitted layers (data streams) are for the same user, and the case (multi-user MIMO) where the layers are for different users.
Single-user MIMO permits spatial multiplexing of four layers using maximum four transmission antennas in a base station. Each layer is not in a one-to-one correspondence with the transmission antennas, and is transmitted from all the transmission antennas using mutually different transmission phase/amplitude control (precoding). By precoding, the layers that are concurrently transmitted are virtually received orthogonally to one another (without mutually interfering) on the receiver side. Therefore, precoding vectors (weights of the transmission antennas) are determined by considering fading variations so that the layers (data streams) that are concurrently transmitted do not interfere mutually and are received in high SINR in LTE terminals. Further, by precoding, it is possible to perform beam formation that actualizes directional transmission for emphasizing desired signals to a particular user terminal.
Multi-user MIMO is actualized by assigning the same resource block (RB) in some subframe to layers of a plurality of user terminals. In the case of multi-user MIMO, the number of layers assigned to each user is limited to “1”.